Prep first
Before you start
- Fix the water source first and dry everything fully.
- Wear gloves, goggles without vents and an N-95 respirator.
- Brush loose mold off outdoors so spores don't scatter.
- Growth soaked into porous wood may not come out, so plan on replacing those boards.
Mold on a deck is a living fungus that settles on damp boards, so the wet spot is the real problem. Scrubbing clears the surface growth, but mildew will return in the same place if the boards keep staying damp. Weathered gray boards may never regain their original color, so judge the result once they've dried.
Before you reach for a mold and mildew remover, check whether the deck is painted or bare wood, since that changes how hard you can scrub. Removing mildew from a painted deck calls for a gentler touch than a raw wood deck. If you plan to paint later, let the boards dry fully first, because trapped moisture will push paint off.
Before you start
What decks & wood fences can take
Safe to use2
- Water
- Oxygen bleach
Use with care5
- Soaking
- Chlorine bleach
- Solvents (alcohol, spot removers)
- Acids (vinegar, lemon)
- Scrubbing and abrasives
6 items
What you'll need
Tap an item to get it on Amazon, or tick what you already have.
- Garden hoseWets the boards, keeps the cleaner wet, and rinses at low pressure.
- Long-handled stiff synthetic-bristle brushScrubs along the grain without metal bristles. (opens Amazon)
- Detergent or plain dish soapCleans the mildew when mixed with warm water. (opens Amazon)
- Garden sprayer (optional)Puts the cleaner on evenly over a large deck.
- Gloves, goggles without vents, N-95 respiratorKeeps spores out of your eyes, mouth and lungs. (opens Amazon)
- Mildew remover labeled for painted or stained woodOnly for stubborn spots or painted boards, after a test. (opens Amazon)
As an Amazon Associate we earn from qualifying purchases.
6 steps
Step by step
Step 1 of 6
Wet the deck first
Spray the boards with the garden hose so they are damp before any cleaner goes on. A damp surface lets the cleaner spread evenly instead of soaking into dry patches. Work in sections you can finish in one pass.
Step 2 of 6
Scrub the mildew clean
Mix a detergent such as dish soap into warm water, and apply it with a garden sprayer or a bucket. Scrub with a long-handled, stiff synthetic-bristle brush, working along the grain of the wood. Put the most effort into the dark patches and the low spots where water sits.
Careful. Steel wool and wire brushes leave iron behind that reacts with the wood and stains it.
Step 3 of 615 minutes
Let the cleaner stay wet
Keep the cleaner wet on the wood for about 15 minutes. If the sun starts to dry it out, mist the boards with the hose, and work one section at a time so no patch sits too long.
Careful. Don't let cleaners or soap dry on the deck, or they will be hard to rinse off.
Step 4 of 6
Rinse the boards well
Rinse the whole area with water from the garden hose on a low-pressure spray. Keep going until no suds come off the wood. Rinse from one end toward the other so the dirty water runs off the boards and away from the cleaned sections.
Step 5 of 624 to 48 hours
Let the deck dry fully
Keep people and furniture off the boards until they are dry all the way through. Wet areas that dry within 24 to 48 hours usually won't grow mold again. If a spot stays damp longer than that, find the shade or leak that keeps it wet and fix that first.
Step 6 of 6
Repeat if mildew returns
Check the same boards after several days or weeks. If mildew has come back, run the scrub, wait and rinse again on those boards only. Let the full drying time pass each time before you call the job finished.
If you'd rather buy than mix
Best products for mold on decks & wood fences
For a painted deck, test the dish soap mix on a hidden board first, and keep it wet for about 15 minutes before you rinse.
These are the everyday supplies the steps above use, in the versions most people buy on Amazon.
Finished the steps? How does it look?
Nice work. Let it dry completely and check it again in good light. Here is what done looks like:
How to tell it's really gone →A faint mark usually needs one more round, not a stronger product.
Heavy buildup usually takes more than one round.
Heavy or long-standing buildup →When to call a professional →How to tell it's working
A clean deck shows the wood's own color with no dark, fuzzy patches left in the grain. If a spot still looks dark once it's dry, the mildew is still there, so scrub that spot again before you move on.
What changes for yours
Adjust for your decks & wood fences
Deck boards before painting
Before you paint, the cleaning steps stay the same, so start with the scrub and rinse. The change is the timing: give the boards the full drying window before any primer or paint goes on, and don't rush it.
A mildew stain that still shows after the rinse should get a second scrub, not a coat of paint. Only paint once the spot looks even and the boards feel dry to the touch.
Weathered gray boards
Gray, weathered boards get the same scrub, but the rinse tells you more. If the wood has grayed so much that no natural color shows, it may stay silver when dry, and that is weathering rather than mildew.
Use a commercial deck cleaner, brightener or restorer only if mildew still shows after the rinse. Leave the silver fibers alone rather than scrubbing harder.
Pressure-treated boards
The deck steps stay the same on pressure-treated boards, since the scrub, the 15-minute wait and the hose rinse suit them. What changes is the stubborn spot: reach for oxygen bleach or 3% hydrogen peroxide before anything stronger.
Give a pressure-treated spot the full drying window before you judge the result, because damp boards can look darker than they are. If the mark still shows once dry, repeat the scrub on that spot only.
Old CCA boards
Old CCA boards are a different job from ordinary mildew, because the wood itself is treated lumber. Before you scrub, check its age and condition, and treat any board that is badly stained as a replacement job.
Plan on replacing old CCA lumber if it's badly stained, and keep the work to the detergent scrub and hose rinse. Wait for full drying before you touch the boards again.
Heavy or long-standing buildup
Old mildew that has sat in the grain for a season or more often needs more than one pass. Start by repeating the detergent scrub with a stiff synthetic-bristle brush, working along the grain, and then rinse it fully.
If the stain has dried into the wood, use a commercial deck cleaner, brightener or restorer made for mildew and dirt. Those products lift the mildew so the wood's own color shows, and oxygen bleach or the 3% hydrogen peroxide sold in drugstores are the bleach-type options to try on a hidden board first.
After the product dries, check the spot in good light. If the wood has grayed so much that its natural color has leached out, it will stay silver when dry and those fibers should be left in place, since that is weathering rather than mildew. If the dark stain still won't come out after a second treatment, it has likely soaked deep into the grain and may not clear completely.
Vinegar, baking soda, peroxide and more
Do home remedies work on mold on decks & wood fences?
Plenty of home remedies get passed around for mildew, but most were written for indoor surfaces, fabric or stone. Here is how each one fits a wood deck, based on the steps above.
Works1Helps2Skip3
Chemical safety
Never mix these
Some products in this guide react dangerously with each other. Use them one at a time and rinse with water in between.
Hydrogen peroxide (3%) + Oxygen bleach (OxiClean)
OxiClean's maker says it should not be mixed with household chemicals other than detergent in the wash.
If they mix by accident: Don't use the mixture; ventilate the room and avoid breathing fumes or touching it; call Poison Control at 1-800-222-1222 for disposal advice or if anyone feels unwell.
What makes it worse
Mistakes to avoid
Don't start with chlorine bleach
On pressure-treated decks, chlorine bleach breaks down the wood's lignin, corrodes metal fasteners and harms nearby plants, and it doesn't kill mildew spores. Chlorine bleach is not a routine mildew fix, so keep it off this job.
Don't mix bleach with ammonia
Ammonia and chlorine bleach form a toxic gas when mixed, so never combine them. Skip any detergent or cleaner that contains ammonia on the deck, and rinse the bucket well between products.
Don't blast soft wood with pressure
Narrow-stream and rotating 'tornado' tips are off limits on decks. Power washing soft woods like western redcedar, or scrubbing them with caustic cleaners, removes wood from the surface.
Don't paint or caulk over mold
Paint laid over mold is likely to peel, so clean and dry the boards before any paint goes on. Do the same before you caulk a gap, and let the wood dry first.
Don't wet-clean an old CCA deck around kids
Wetting old CCA wood raises the arsenic at its surface, and bleach releases chromate from it. Keep children and pets off the wet boards until they are dry.
When to call a professional
If mildew covers more than about 10 square feet, or came from sewage or contaminated water, stop and bring in an experienced professional and follow the official remediation guidance. An old CCA deck is better replaced than cut or burned, because the sawdust and smoke carry arsenic.
Questions
Can I clean mildew off a painted deck?
Yes, with a mildew remover labeled safe for painted or stained wood. Test it on a hidden board first, then rinse and dry as usual. Allow the full drying time before you recoat.
Will mildew come back after cleaning?
It can, if the boards keep getting wet. Mildew that returns in several days or weeks means the wood is still damp, so repeat the cleaning and fix the water source.
Can I use vinegar on my deck?
No. Vinegar is used for indoor hard surfaces, so use a detergent scrub and hose rinse on a deck. It's used for indoor hard surfaces, so use the detergent scrub and hose rinse outside.
Does mildew damage the wood itself?
It can. Mold may cause staining and cosmetic damage that can't always be reversed, so a faint mark can remain after a good clean.
Do I need to sand the deck after cleaning mildew?
Usually not for mildew alone. Sand only if old stain or sealer is coming off, and only lightly once the boards are dry, after using a stain and sealer remover and a brightener.
Do I need protective gear for this?
Yes. Wear gloves, goggles without vents and an N-95 respirator while you brush and scrub, so you don't breathe in or touch spores.
Mold on other materials
Other stains on decks & wood fences
Start from the bigger picture
How we know this
Every method, amount and warning here comes from university extension services, industry bodies and product makers. We read them, check every step for unsafe combinations and changed amounts, and write it up so you can follow it.